This is only tested with a Rogue.
Queue for battle and let the boss aggro you, but don't attack him.
Request invite from a friend on other realm accept it and vanish, you should be at his realm inside the arena with someone else fighting, if he wins you get the win to.
If the person is about to lose leave the group and get another chance in your server if someone fighting.
Could Also work with other classes if you ready the invite beforehand and accept as you enter the arena.